This invention relates to the protection of superalloys to be used at elevated temperatures, and, more particularly, to coatings applied to such superalloys.
One of the most demanding materials applications in current technology is found in the hot-stage components used in aircraft jet engines. The higher the operating temperature of an engine, the greater its efficiency, and the more power it can produce from each gallon of fuel. There is therefore an incentive to operate such engines at as high a temperature as possible. The primary limitation on the operating temperatures of a jet engine is the materials used in the hottest regions of the engine, such as gas turbine blades and vanes.
There has been much research to develop materials that can be used in high temperature engine applications. The currently most popular and successful of such materials are the nickel-base superalloys, which are alloys of nickel with additions of a number of other elements such as, for example, chromium, cobalt, aluminum, and tantalum. The compositions of these superalloys are carefully engineered to maintain their strength and other mechanical properties even during use at the high temperature of engine operation, which is in the neighborhood of 2000.degree. F. or more.
The materials used in the jet engines must operate at high temperatures, but additionally are subjected to oxidative and corrosive conditions. Oxidation of materials such as nickel and many of its alloys is rapid at engine operating temperatures. The engine components are also subjected to corrosive attack by chemicals in the burned fuel, as well as ingested agents such as salt that might be drawn into the engine as it operates near an ocean. The materials that have the best mechanical properties at high temperatures often are not as resistant to oxidation and corrosion as other materials, and there is an ongoing search for materials that offer a compromise between the best mechanical properties and the best oxidation and corrosion resistance.
High operating temperatures can also be achieved by other techniques not related directly to the alloy compositions used in the components. For example, control of grain structures and preparation of components as single crystals may result in improved properties. Cooling passages may be provided in the components, and cooling air passed through them to lower their actual operating temperature.
In another approach which is the primary focus of the present invention, a thin protective metallic coating is deposited upon the component. The coating protects the substrate from oxidation and corrosion damage The coating must be adherent to the superalloy substrate and must remain adherent through many cycles of heating to the operating temperature and then cooling back to a lower temperature when the engine is idling or turned off. Because materials of different compositions have different coefficients of thermal expansion, cycles of heating and cooling tend to cause the coating to crack and/or spall off, which results in the exposure of the superalloy substrate to the environment, and subsequent deterioration of the substrate.
To accommodate the strains imposed by the thermal cycling, the thin coatings have historically been made of materials that are relatively weak and ductile at operating temperatures. In theory, such a coating can plastically deform either in tension or compression to remain adherent to the surface of the substrate as the substrate is heated and cooled. Most coatings for nickel-base superalloys have been made of alloys of nickel, chromium, aluminum, and yttrium, which are termed NiCrAlY alloys, and nickel, cobalt, chromium, aluminum, and yttrium, which are termed NiCoCrAlY alloys. The term MCrAlX, where M represents nickel, cobalt, iron or some combination thereof and X represents yttrium, hafnium, tantalum, silicon or some combination thereof, is a widely used generic description for this type of alloy. While such alloys contain many of the same elements as the substrate materials, the proportions of those elements have been adjusted to enhance oxidation and corrosion resistance rather than mechanical properties. They therefore lack the strength to serve as the structural components themselves, but serve well as protective coatings.
However, recent engine operating experience has shown that such coatings may be too weak for some applications characterized by large amounts of cyclic plastic strain. Under such conditions a weak coating is vulnerable to wrinkling and cracking. The resulting cracks may extend through the coating to the substrate, and in those locations the substrate is subject to the same deterioration as if it had not been coated with the protective coating. Thus, even though the coatings do not necessarily need the strength to function as the primary load bearing structural members, they must be sufficiently strong to resist cracking or failure induced by thermal fatigue as the coated substrate is repeatedly heated and cooled. For this reason, as superalloy substrates of increased strength and operating temperatures are developed, it is necessary that the strength of coatings used on these substrates also be improved. At the present time, improved strengths of the coating materials are needed so that the coatings will not fail prematurely, long before the substrates would fail.
Therefore, there is an ongoing need for improved metallic coating materials that can protect the substrates against oxidation and corrosion damage for extended periods of cyclic loading. The present invention fulfills this need, and further provides related advantages.